Ognl对象图导航语言 源码

// --------------------------------------------------------------------------
// Copyright (c) 1998-2004, Drew Davidson and Luke Blanshard
// All rights reserved.
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package ognl;

import ognl.enhance.ExpressionAccessor;

import java.io.StringReader;
import java.util.Map;

/**
 * <P>
 * This class provides static methods for parsing and interpreting OGNL expressions.
 * </P>
 * <P>
 * The simplest use of the Ognl class is to get the value of an expression from an object, without
 * extra context or pre-parsing.
 * </P>
 *
 * <PRE>
 *
 * import ognl.Ognl; import ognl.OgnlException; try { result = Ognl.getValue(expression, root); }
 * catch (OgnlException ex) { // Report error or recover }
 *
 * </PRE>
 *
 * <P>
 * This will parse the expression given and evaluate it against the root object given, returning the
 * result. If there is an error in the expression, such as the property is not found, the exception
 * is encapsulated into an {@link ognl.OgnlException OgnlException}.
 * </P>
 * <P>
 * Other more sophisticated uses of Ognl can pre-parse expressions. This provides two advantages: in
 * the case of user-supplied expressions it allows you to catch parse errors before evaluation and
 * it allows you to cache parsed expressions into an AST for better speed during repeated use. The
 * pre-parsed expression is always returned as an <CODE>Object</CODE> to simplify use for programs
 * that just wish to store the value for repeated use and do not care that it is an AST. If it does
 * care it can always safely cast the value to an <CODE>AST</CODE> type.
 * </P>
 * <P>
 * The Ognl class also takes a <I>context map</I> as one of the parameters to the set and get
 * methods. This allows you to put your own variables into the available namespace for OGNL
 * expressions. The default context contains only the <CODE>#root</CODE> and <CODE>#context</CODE>
 * keys, which are required to be present. The <CODE>addDefaultContext(Object, Map)</CODE> method
 * will alter an existing <CODE>Map</CODE> to put the defaults in. Here is an example that shows
 * how to extract the <CODE>documentName</CODE> property out of the root object and append a
 * string with the current user name in parens:
 * </P>
 *
 * <PRE>
 *
 * private Map context = new HashMap(); public void setUserName(String value) {
 * context.put("userName", value); } try { // get value using our own custom context map result =
 * Ognl.getValue("documentName + \" (\" + ((#userName == null) ? \"&lt;nobody&gt;\" : #userName) +
 * \")\"", context, root); } catch (OgnlException ex) { // Report error or recover }
 *
 * </PRE>
 *
 * @author Luke Blanshard (blanshlu@netscape.net)
 * @author Drew Davidson (drew@ognl.org)
 * @version 27 June 1999
 */
public abstract class Ognl
{


    /**
     * Parses the given OGNL expression and returns a tree representation of the expression that can
     * be used by <CODE>Ognl</CODE> static methods.
     *
     * @param expression
     *            the OGNL expression to be parsed
     * @return a tree representation of the expression
     * @throws ExpressionSyntaxException
     *             if the expression is malformed
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static Object parseExpression(String expression)
            throws OgnlException
    {
        try {
            OgnlParser parser = new OgnlParser(new StringReader(expression));
            return parser.topLevelExpression();
        } catch (ParseException e) {
            throw new ExpressionSyntaxException(expression, e);
        } catch (TokenMgrError e) {
            throw new ExpressionSyntaxException(expression, e);
        }
    }

    /**
     * Parses and compiles the given expression using the {@link ognl.enhance.OgnlExpressionCompiler} returned
     * from {@link ognl.OgnlRuntime#getCompiler()}.
     *
     * @param context
     *          The context to use.
     * @param root
     *          The root object for the given expression.
     * @param expression
     *          The expression to compile.
     *
     * @return The node with a compiled accessor set on {@link ognl.Node#getAccessor()} if compilation
     *          was successfull. In instances where compilation wasn't possible because of a partially null
     *          expression the {@link ExpressionAccessor} instance may be null and the compilation of this expression
     *          still possible at some as yet indertermined point in the future.
     *
     * @throws Exception If a compilation error occurs.
     */
    public static Node compileExpression(OgnlContext context, Object root, String expression)
            throws Exception
    {
        Node expr = (Node)Ognl.parseExpression(expression);

        OgnlRuntime.compileExpression(context, expr, root);

        return expr;
    }

    /**
     * Creates and returns a new standard naming context for evaluating an OGNL expression.
     *
     * @param root
     *            the root of the object graph
     * @return a new Map with the keys <CODE>root</CODE> and <CODE>context</CODE> set
     *         appropriately
     */
    public static Map createDefaultContext(Object root)
    {
        return addDefaultContext(root, null, null, null, new OgnlContext());
    }

    /**
     * Creates and returns a new standard naming context for evaluating an OGNL expression.
     *
     * @param root
     *          The root of the object graph.
     * @param classResolver
     *          The resolver used to instantiate {@link Class} instances referenced in the expression.
     *
     * @return a new OgnlContext with the keys <CODE>root</CODE> and <CODE>context</CODE> set
     *         appropriately
     */
    public static Map createDefaultContext(Object root, ClassResolver classResolver)
    {
        return addDefaultContext(root, classResolver, null, null, new OgnlContext());
    }

    /**
     * Creates and returns a new standard naming context for evaluating an OGNL expression.
     *
     * @param root
     *          The root of the object graph.
     * @param classResolver
     *          The resolver used to instantiate {@link Class} instances referenced in the expression.
     * @param converter
     *          Converter used to convert return types of an expression in to their desired types.
     *
     * @return a new Map with the keys <CODE>root</CODE> and <CODE>context</CODE> set
     *         appropriately
     */
    public static Map createDefaultContext(Object root, ClassResolver classResolver, TypeConverter converter)
    {
        return addDefaultContext(root, classResolver, converter, null, new OgnlContext());
    }

    /**
     * Creates and returns a new standard naming context for evaluating an OGNL expression.
     *
     * @param root
     *          The root of the object graph.
     * @param classResolver
     *          The resolver used to instantiate {@link Class} instances referenced in the expression.
     * @param converter
     *          Converter used to convert return types of an expression in to their desired types.
     * @param memberAccess
     *          Java security handling object to determine semantics for accessing normally private/protected
     *          methods / fields.
     * @return a new Map with the keys <CODE>root</CODE> and <CODE>context</CODE> set
     *         appropriately
     */
    public static Map createDefaultContext(Object root, ClassResolver classResolver,
                                           TypeConverter converter, MemberAccess memberAccess)
    {
        return addDefaultContext(root, classResolver, converter, memberAccess, new OgnlContext());
    }

    /**
     * Appends the standard naming context for evaluating an OGNL expression into the context given
     * so that cached maps can be used as a context.
     *
     * @param root
     *            the root of the object graph
     * @param context
     *            the context to which OGNL context will be added.
     * @return Context Map with the keys <CODE>root</CODE> and <CODE>context</CODE> set
     *         appropriately
     */
    public static Map addDefaultContext(Object root, Map context)
    {
        return addDefaultContext(root, null, null, null, context);
    }

    /**
     * Appends the standard naming context for evaluating an OGNL expression into the context given
     * so that cached maps can be used as a context.
     *
     * @param root
     *          The root of the object graph.
     * @param classResolver
     *          The resolver used to instantiate {@link Class} instances referenced in the expression.
     * @param context
     *          The context to which OGNL context will be added.
     *
     * @return Context Map with the keys <CODE>root</CODE> and <CODE>context</CODE> set
     *         appropriately
     */
    public static Map addDefaultContext(Object root, ClassResolver classResolver, Map context)
    {
        return addDefaultContext(root, classResolver, null, null, context);
    }

    /**
     * Appends the standard naming context for evaluating an OGNL expression into the context given
     * so that cached maps can be used as a context.
     *
     * @param root
     *          The root of the object graph.
     * @param classResolver
     *          The resolver used to instantiate {@link Class} instances referenced in the expression.
     * @param converter
     *          Converter used to convert return types of an expression in to their desired types.
     * @param context
     *          The context to which OGNL context will be added.
     *
     * @return Context Map with the keys <CODE>root</CODE> and <CODE>context</CODE> set
     *         appropriately
     */
    public static Map addDefaultContext(Object root, ClassResolver classResolver,
                                        TypeConverter converter, Map context)
    {
        return addDefaultContext(root, classResolver, converter, null, context);
    }

    /**
     * Appends the standard naming context for evaluating an OGNL expression into the context given
     * so that cached maps can be used as a context.
     *
     * @param root
     *            the root of the object graph
     * @param classResolver
     *            The class loading resolver that should be used to resolve class references.
     * @param converter
     *            The type converter to be used by default.
     * @param memberAccess

     *              Definition for handling private/protected access.
     * @param context
     *              Default context to use, if not an {@link OgnlContext} will be dumped into
     *              a new {@link OgnlContext} object.
     * @return Context Map with the keys <CODE>root</CODE> and <CODE>context</CODE> set
     *         appropriately
     */
    public static Map addDefaultContext(Object root, ClassResolver classResolver,
                                        TypeConverter converter, MemberAccess memberAccess, Map context)
    {
        OgnlContext result;

        if (!(context instanceof OgnlContext)) {
            result = new OgnlContext();
            result.setValues(context);
        } else {
            result = (OgnlContext) context;
        }
        if (classResolver != null) {
            result.setClassResolver(classResolver);
        }
        if (converter != null) {
            result.setTypeConverter(converter);
        }
        if (memberAccess != null) {
            result.setMemberAccess(memberAccess);
        }

        result.setRoot(root);
        return result;
    }

    /**
     * Configures the {@link ClassResolver} to use for the given context.  Will be used during
     * expression parsing / execution to resolve class names.
     *
     * @param context
     *          The context to place the resolver.
     * @param classResolver
     *          The resolver to use to resolve classes.
     */
    public static void setClassResolver(Map context, ClassResolver classResolver)
    {
        context.put(OgnlContext.CLASS_RESOLVER_CONTEXT_KEY, classResolver);
    }

    /**
     * Gets the previously stored {@link ClassResolver} for the given context - if any.
     *
     * @param context
     *          The context to get the configured resolver from.
     *
     * @return The resolver instance, or null if none found. 
     */
    public static ClassResolver getClassResolver(Map context)
    {
        return (ClassResolver) context.get(OgnlContext.CLASS_RESOLVER_CONTEXT_KEY);
    }

    /**
     * Configures the type converter to use for a given context. This will be used
     * to convert into / out of various java class types.
     *
     * @param context
     *          The context to configure it for.
     * @param converter
     *          The converter to use.
     */
    public static void setTypeConverter(Map context, TypeConverter converter)
    {
        context.put(OgnlContext.TYPE_CONVERTER_CONTEXT_KEY, converter);
    }

    /**
     * Gets the currently configured {@link TypeConverter} for the given context - if any.
     *
     * @param context
     *          The context to get the converter from.
     *
     * @return The converter - or null if none found.
     */
    public static TypeConverter getTypeConverter(Map context)
    {
        return (TypeConverter) context.get(OgnlContext.TYPE_CONVERTER_CONTEXT_KEY);
    }

    /**
     * Configures the specified context with a {@link MemberAccess} instance for
     * handling field/method protection levels.
     *
     * @param context
     *          The context to configure.
     * @param memberAccess
     *          The access resolver to configure the context with.
     */
    public static void setMemberAccess(Map context, MemberAccess memberAccess)
    {
        context.put(OgnlContext.MEMBER_ACCESS_CONTEXT_KEY, memberAccess);
    }

    /**
     * Gets the currently stored {@link MemberAccess} object for the given context - if any.
     *
     * @param context
     *          The context to get the object from.
     * 
     * @return The configured {@link MemberAccess} instance in the specified context - or null if none found.
     */
    public static MemberAccess getMemberAccess(Map context)
    {
        return (MemberAccess) context.get(OgnlContext.MEMBER_ACCESS_CONTEXT_KEY);
    }

    /**
     * Sets the root object to use for all expressions in the given context - doesn't necessarily replace
     * root object instances explicitly passed in to other expression resolving methods on this class.
     *
     * @param context
     *          The context to store the root object in.
     * @param root
     *          The root object.
     */
    public static void setRoot(Map context, Object root)
    {
        context.put(OgnlContext.ROOT_CONTEXT_KEY, root);
    }

    /**
     * Gets the stored root object for the given context - if any.
     *
     * @param context
     *          The context to get the root object from.
     * 
     * @return The root object - or null if none found.
     */
    public static Object getRoot(Map context)
    {
        return context.get(OgnlContext.ROOT_CONTEXT_KEY);
    }

    /**
     * Gets the last {@link Evaluation} executed on the given context.
     *
     * @param context
     *          The context to get the evaluation from.
     * 
     * @return The {@link Evaluation} - or null if none was found.
     */
    public static Evaluation getLastEvaluation(Map context)
    {
        return (Evaluation) context.get(OgnlContext.LAST_EVALUATION_CONTEXT_KEY);
    }

    /**
     * Evaluates the given OGNL expression tree to extract a value from the given root object. The
     * default context is set for the given context and root via <CODE>addDefaultContext()</CODE>.
     *
     * @param tree
     *            the OGNL expression tree to evaluate, as returned by parseExpression()
     * @param context
     *            the naming context for the evaluation
     * @param root
     *            the root object for the OGNL expression
     * @return the result of evaluating the expression
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static Object getValue(Object tree, Map context, Object root)
            throws OgnlException
    {
        return getValue(tree, context, root, null);
    }

    /**
     * Evaluates the given OGNL expression tree to extract a value from the given root object. The
     * default context is set for the given context and root via <CODE>addDefaultContext()</CODE>.
     *
     * @param tree
     *            the OGNL expression tree to evaluate, as returned by parseExpression()
     * @param context
     *            the naming context for the evaluation
     * @param root
     *            the root object for the OGNL expression
     * @param resultType
     *            the converted type of the resultant object, using the context's type converter
     * @return the result of evaluating the expression
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static Object getValue(Object tree, Map context, Object root, Class resultType)
            throws OgnlException
    {
        Object result;
        OgnlContext ognlContext = (OgnlContext) addDefaultContext(root, context);

        Node node = (Node)tree;

        if (node.getAccessor() != null)
            result =  node.getAccessor().get(ognlContext, root);
        else
            result = node.getValue(ognlContext, root);

        if (resultType != null) {
            result = getTypeConverter(context).convertValue(context, root, null, null, result, resultType);
        }
        return result;
    }

    /**
     * Gets the value represented by the given pre-compiled expression on the specified root 
     * object.
     *
     * @param expression
     *          The pre-compiled expression, as found in {@link Node#getAccessor()}. 
     * @param context
     *          The ognl context.
     * @param root
     *          The object to retrieve the expression value from.
     * @return
     *          The value.
     */
    public static Object getValue(ExpressionAccessor expression, OgnlContext context, Object root)
    {
        return expression.get(context, root);
    }

    /**
     * Gets the value represented by the given pre-compiled expression on the specified root 
     * object.
     *
     * @param expression
     *          The pre-compiled expression, as found in {@link Node#getAccessor()}. 
     * @param context
     *          The ognl context.
     * @param root
     *          The object to retrieve the expression value from.
     * @param resultType
     *          The desired object type that the return value should be converted to using the {@link #getTypeConverter(java.util.Map)} }.
     * @return
     *          The value.
     */
    public static Object getValue(ExpressionAccessor expression, OgnlContext context,
                                  Object root, Class resultType)
    {
        return getTypeConverter(context).convertValue(context, root, null, null, expression.get(context, root), resultType);
    }

    /**
     * Evaluates the given OGNL expression to extract a value from the given root object in a given
     * context
     *
     * @see #parseExpression(String)
     * @see #getValue(Object,Object)
     * @param expression
     *            the OGNL expression to be parsed
     * @param context
     *            the naming context for the evaluation
     * @param root
     *            the root object for the OGNL expression
     * @return the result of evaluating the expression
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static Object getValue(String expression, Map context, Object root)
            throws OgnlException
    {
        return getValue(expression, context, root, null);
    }

    /**
     * Evaluates the given OGNL expression to extract a value from the given root object in a given
     * context
     *
     * @see #parseExpression(String)
     * @see #getValue(Object,Object)
     * @param expression
     *            the OGNL expression to be parsed
     * @param context
     *            the naming context for the evaluation
     * @param root
     *            the root object for the OGNL expression
     * @param resultType
     *            the converted type of the resultant object, using the context's type converter
     * @return the result of evaluating the expression
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static Object getValue(String expression, Map context, Object root, Class resultType)
            throws OgnlException
    {
        return getValue(parseExpression(expression), context, root, resultType);
    }

    /**
     * Evaluates the given OGNL expression tree to extract a value from the given root object.
     *
     * @param tree
     *            the OGNL expression tree to evaluate, as returned by parseExpression()
     * @param root
     *            the root object for the OGNL expression
     * @return the result of evaluating the expression
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static Object getValue(Object tree, Object root)
            throws OgnlException
    {
        return getValue(tree, root, null);
    }

    /**
     * Evaluates the given OGNL expression tree to extract a value from the given root object.
     *
     * @param tree
     *            the OGNL expression tree to evaluate, as returned by parseExpression()
     * @param root
     *            the root object for the OGNL expression
     * @param resultType
     *            the converted type of the resultant object, using the context's type converter
     * @return the result of evaluating the expression
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static Object getValue(Object tree, Object root, Class resultType)
            throws OgnlException
    {
        return getValue(tree, createDefaultContext(root), root, resultType);
    }

    /**
     * Convenience method that combines calls to <code> parseExpression </code> and
     * <code> getValue</code>.
     *
     * @see #parseExpression(String)
     * @see #getValue(Object,Object)
     * @param expression
     *            the OGNL expression to be parsed
     * @param root
     *            the root object for the OGNL expression
     * @return the result of evaluating the expression
     * @throws ExpressionSyntaxException
     *             if the expression is malformed
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static Object getValue(String expression, Object root)
            throws OgnlException
    {
        return getValue(expression, root, null);
    }

    /**
     * Convenience method that combines calls to <code> parseExpression </code> and
     * <code> getValue</code>.
     *
     * @see #parseExpression(String)
     * @see #getValue(Object,Object)
     * @param expression
     *            the OGNL expression to be parsed
     * @param root
     *            the root object for the OGNL expression
     * @param resultType
     *            the converted type of the resultant object, using the context's type converter
     * @return the result of evaluating the expression
     * @throws ExpressionSyntaxException
     *             if the expression is malformed
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static Object getValue(String expression, Object root, Class resultType)
            throws OgnlException
    {
        return getValue(parseExpression(expression), root, resultType);
    }

    /**
     * Evaluates the given OGNL expression tree to insert a value into the object graph rooted at
     * the given root object. The default context is set for the given context and root via <CODE>addDefaultContext()</CODE>.
     *
     * @param tree
     *            the OGNL expression tree to evaluate, as returned by parseExpression()
     * @param context
     *            the naming context for the evaluation
     * @param root
     *            the root object for the OGNL expression
     * @param value
     *            the value to insert into the object graph
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static void setValue(Object tree, Map context, Object root, Object value)
            throws OgnlException
    {
        OgnlContext ognlContext = (OgnlContext) addDefaultContext(root, context);
        Node n = (Node) tree;

        if (n.getAccessor() != null) {
            n.getAccessor().set(ognlContext, root, value);
            return;
        }

        n.setValue(ognlContext, root, value);
    }

    /**
     * Sets the value given using the pre-compiled expression on the specified root 
     * object.
     *
     * @param expression
     *          The pre-compiled expression, as found in {@link Node#getAccessor()}. 
     * @param context
     *          The ognl context.
     * @param root
     *          The object to set the expression value on.
     * @param value
     *          The value to set.
     */
    public static void setValue(ExpressionAccessor expression, OgnlContext context,
                                Object root, Object value)
    {
        expression.set(context, root, value);
    }

    /**
     * Evaluates the given OGNL expression to insert a value into the object graph rooted at the
     * given root object given the context.
     *
     * @param expression
     *            the OGNL expression to be parsed
     * @param root
     *            the root object for the OGNL expression
     * @param context
     *            the naming context for the evaluation
     * @param value
     *            the value to insert into the object graph
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static void setValue(String expression, Map context, Object root, Object value)
            throws OgnlException
    {
        setValue(parseExpression(expression), context, root, value);
    }

    /**
     * Evaluates the given OGNL expression tree to insert a value into the object graph rooted at
     * the given root object.
     *
     * @param tree
     *            the OGNL expression tree to evaluate, as returned by parseExpression()
     * @param root
     *            the root object for the OGNL expression
     * @param value
     *            the value to insert into the object graph
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static void setValue(Object tree, Object root, Object value)
            throws OgnlException
    {
        setValue(tree, createDefaultContext(root), root, value);
    }

    /**
     * Convenience method that combines calls to <code> parseExpression </code> and
     * <code> setValue</code>.
     *
     * @see #parseExpression(String)
     * @see #setValue(Object,Object,Object)
     * @param expression
     *            the OGNL expression to be parsed
     * @param root
     *            the root object for the OGNL expression
     * @param value
     *            the value to insert into the object graph
     * @throws ExpressionSyntaxException
     *             if the expression is malformed
     * @throws MethodFailedException
     *             if the expression called a method which failed
     * @throws NoSuchPropertyException
     *             if the expression referred to a nonexistent property
     * @throws InappropriateExpressionException
     *             if the expression can't be used in this context
     * @throws OgnlException
     *             if there is a pathological environmental problem
     */
    public static void setValue(String expression, Object root, Object value)
            throws OgnlException
    {
        setValue(parseExpression(expression), root, value);
    }

    /**
     * Checks if the specified {@link Node} instance represents a constant
     * expression.
     *
     * @param tree
     *          The {@link Node} to check.
     * @param context
     *          The context to use.
     *
     * @return True if the node is a constant - false otherwise.
     * @throws OgnlException  If an error occurs checking the expression.
     */
    public static boolean isConstant(Object tree, Map context)
            throws OgnlException
    {
        return ((SimpleNode) tree).isConstant((OgnlContext) addDefaultContext(null, context));
    }

     /**
     * Checks if the specified expression represents a constant expression.
     *
     * @param expression
     *          The expression to check.
     * @param context
     *          The context to use.
     *
     * @return True if the node is a constant - false otherwise.
     * @throws OgnlException  If an error occurs checking the expression.
     */
    public static boolean isConstant(String expression, Map context)
            throws OgnlException
    {
        return isConstant(parseExpression(expression), context);
    }

    /**
     * Same as {@link #isConstant(Object, java.util.Map)} - only the {@link Map} context
     * is created for you.
     *
     * @param tree
     *          The {@link Node} to check.
     * 
     * @return True if the node represents a constant expression - false otherwise.
     * @throws OgnlException If an exception occurs.
     */
    public static boolean isConstant(Object tree)
            throws OgnlException
    {
        return isConstant(tree, createDefaultContext(null));
    }

    /**
     * Same as {@link #isConstant(String, java.util.Map)} - only the {@link Map}
     * instance is created for you.
     *
     * @param expression
     *          The expression to check.
     *
     * @return True if the expression represents a constant - false otherwise.
     * @throws OgnlException If an exception occurs.
     */
    public static boolean isConstant(String expression)
            throws OgnlException
    {
        return isConstant(parseExpression(expression), createDefaultContext(null));
    }

    public static boolean isSimpleProperty(Object tree, Map context)
            throws OgnlException
    {
        return ((SimpleNode) tree).isSimpleProperty((OgnlContext) addDefaultContext(null, context));
    }

    public static boolean isSimpleProperty(String expression, Map context)
            throws OgnlException
    {
        return isSimpleProperty(parseExpression(expression), context);
    }

    public static boolean isSimpleProperty(Object tree)
            throws OgnlException
    {
        return isSimpleProperty(tree, createDefaultContext(null));
    }

    public static boolean isSimpleProperty(String expression)
            throws OgnlException
    {
        return isSimpleProperty(parseExpression(expression), createDefaultContext(null));
    }

    public static boolean isSimpleNavigationChain(Object tree, Map context)
            throws OgnlException
    {
        return ((SimpleNode) tree).isSimpleNavigationChain((OgnlContext) addDefaultContext(null, context));
    }

    public static boolean isSimpleNavigationChain(String expression, Map context)
            throws OgnlException
    {
        return isSimpleNavigationChain(parseExpression(expression), context);
    }

    public static boolean isSimpleNavigationChain(Object tree)
            throws OgnlException
    {
        return isSimpleNavigationChain(tree, createDefaultContext(null));
    }

    public static boolean isSimpleNavigationChain(String expression)
            throws OgnlException
    {
        return isSimpleNavigationChain(parseExpression(expression), createDefaultContext(null));
    }

    /** You can't make one of these. */
    private Ognl()
    {
    }
}